In a continuous high-speed electric plating apparatus such as a zinc plating apparatus and a tin plating apparatus for steel plates, an insoluble lead electrode and a insoluble lead alloy electrode have been used in the past. In these types of apparatus, the effective electrode area of the electrode is very large, for example from 1 to 3 m.sup.2.
Also, in a metal foil continuous electrolytically producing apparatus such as a copper foil electrolytic producing apparatus, a lead alloy electrode has been used for a long time as an anode facing cylindrical cathod. The anode surrounds 1/4 of the circumference of the cylindrical cathod having a diameter of 3 meters and a width of from 1.5 to 2 meters and the size of the anode is from 3.5 to 4 m.sup.2. A lead alloy electode has a low melting point and can be easily worked. Even when the size of the anode is large, the anode can be easily welded and the form can be easily adjusted at a place where a metal foil producing apparatus is disposed. Thus, problems in working the electrode are relatively low.
However, since it is difficult to uniformly solidify a molten electrode material for forming a large electrode and in particular, it is impossible to uniformly solidfy the molten electrode material at the location where an electrolytic apparatus is disposed, it is difficult to obtain a uniform alloy composition at the active portion of the electrode. As a result, it is actually impossible to provide the electrode surface with a uniform electrode potential.
That is, the oxygen generating potential of the lead alloy electrode in sulfuric acid deviates from 1.8 volts to 2.2 volts to NHE at 60.degree. C. and 20 A/dm.sup.2, and also a lead-silver alloy electrode having an excellent corrosion resistance usually deviates from 100 mV to 200 mV with a slight difference in silver content.
Accordingly, the electrode which is used for the purpose of passing an electric current as a counter electrode to a cathode which is a working electrode is easily usable but even the electrode has a large disadvantage for obtaining a high accuracy as the elecrode for an electric plating apparatus or a metal foil producing apparatus which must have passing electric current uniformity.
Also, since the consumption of these insoluble elecrodes on electrolysis is very large as several mg/Ah and the change in the form of the electrode surface is large, there is a problem that maintenance must be frequently carried out for correction and there are problems that lead to contamination of the electrolyte by consumed components which are intermixed in the products as metallic lead, lead ions, lead sulfate, and/or lead oxide. This reduces the quality of the products and also causes environmental pollution if the waste solution is not treated.
Thus, for solving these problems, recently, an insoluble metal elecrtode formed by coating the surface of a rare metal such as titanium with an electrically conductive electrode material such as a platinum group metal has been used.
A platinum-plated titanium electrode has been used as an insoluble metal electrode for a long time. The consumption of platinum is from several mg/kAh to several tens of mg/kAh, which is far less than that of a lead alloy. However, the consumed amount of the electrode is larger than that of a platinum-plated titanium electrode which is used for a general electrolysis of an aqueous solution. Thus, there is a problem that the electrode with a platinum coating of an ordinary thickness of from 3 to 5 .mu.m, i.e., having a platinum coated amount of from 60 to 100 g/m.sup.2, has a very short electrode life.
On the other hand, an oxide series insoluble metal electrode has a long life, the potential thereof is low as from 300 mV to 500 mV as compared with platinum and is considered to be an ideal electrode. In this case, it is necessary for the oxide series insoluble electrode coating to be uniformly coated on the entire surface of a large electrode of larger than 1 square meter for obtaining a uniform electrode potential and the potential loss due to electrical conductive resistance is reduced. Hence, increasing the thickness of the electrode base plate to reduce the electrical conductive resistance has been the practice in the past. For example, when titanium is used as an electrode base plate, a base plate having a thickness of from 25 mm to 40 mm is used.
The oxide electrode coating is formed by coating a solution of a metal capable of providing an oxide coating by thermal decomposition and thermally decomposing the metal in an oxidative atmosphere as descirbed in U.S. Pat. Nos. 3,632,498 and 3,711,385. The thermal decomposition method provides the ability to form the oxide electrode coating with a desired thickness by repeating the operation of coating the metal solution and thermally decomposing the metal. However, in this case, heating and cooling of a thick and large electrode plate must be repeatedly carried out. Hence a very long time and must labor is required for the production of the electrode.
Also, in the insoluble metal electrode, when a very small part of the electrode coating is deteriorated during the use, the metal foil or the plated product obtained becomes nonuniform. Hence, there is a problem that in such a case, not a partial treatment of the electrode coating but the regenerating treatment of the entire surface is necessary.